Fast applications growth and network expansion speed up the development of the Virtual Private Network (VPN) technology to save the IP address resources and secure private networks, and thus promotes the fast development of tunneling technologies.
Among the existing tunneling implementations, some bidirectional tunnels are configured with the corresponding logical interfaces at both ends to transmit traffic in between, thus facilitating the deployment of some network applications. For example, you can run routing, multicast, and LDP protocols on the directly-connected logical interfaces of a bidirectional tunnel, making the traffic pass the tunnel easily; for another example, a tunnel spans an IPv4 public network, but the logical interfaces of the tunnel can run IPv6 protocols to implement IPv6 over IPv4; additionally, the logical tunnel interfaces can be bound to VPNs and run routing protocols, thus implementing interconnection between multiple VPN nodes.
The logical interfaces of the existing bidirectional tunnels operate as Layer 3 interfaces at the network layer. FIG. 1 is the schematic diagram illustrating application instance I where the logical interfaces of a bidirectional tunnel operate as Layer 3 interfaces. As shown in FIG. 1, a bidirectional tunnel spanning the public network is set up between two PE devices, and the logical interfaces at both ends of the tunnel are bound to the VPN routing and forwarding (VRF) tables of the private networks. In this way, there is a pair of directly-connected P2P interfaces between the VRF tables of the private networks. By running a routing protocol on the logical interfaces of the bidirectional tunnel, you can make the private networks reachable to each other over the tunnel, thus implementing a simple VPN scheme. By running the LDP protocol on the logical interfaces of the bidirectional tunnel, you can easily get the MPLS LDP LSPs of different VPN nodes, and thus easily deploy the networking scheme of the service provider. By running a multicast routing protocol on the logical interfaces of the bidirectional tunnel, you can also deploy a simple multicast VPN.
In a similar way, you can configure another bidirectional tunnel on the same public network, and run routing, multicast, and LDP protocols on it.
FIG. 2 is the schematic diagram illustrating application instance II where the logical interfaces of a bidirectional tunnel operate as Layer 3 interfaces. You can use the bidirectional tunnel and its logical interfaces to implement networking schemes such as IPv6 over IPv4 and IPv4 over IPv6. To do that in FIG. 2, you need only configure IPv6 addresses for the logical interfaces and run IPv6 routing protocols on them.
However, the existing bidirectional tunnel logical interfaces as previously understood by the inventor of the present invention can operate only as Layer 3 interfaces. At Layer 2, Layer 2 tunneling is required to span a public network.